The world's principal industrial processes for fixing atmospheric nitrogen are the Haber-Bosch process and similar methodologies which combine molecular nitrogen with hydrogen over solid catalysts at high temperatures and pressures to form ammonia. These processes require relatively large amounts of energy, are technically sophisticated, and are primarily based on the use of fossil fuels (for instance coal or natural gas) in the production of hydrogen. By their very nature and chemical requirements, such methodologies are appropriate only for economies of large scale which can provide the reactants in volume, central production facilities, and the requisite distribution systems for effective use of the process.
Alternatives to large scale industrial methods for production of ammonia have been sought with the result that considerable chemical research has been directed towards finding economically viable and less energy-consuming methods. One approach has been the use of metal oxide catalysts and gaseous nitrogen from the air in the absence of photoenergy. Exemplifying this approach is U.S. Pat. No. 2,500,008 which describes the synthesis of ammonia from a mixture of hydrogen and nitrogen which is combined with a finely divided iron oxide catalyst and subjected to ultrasonic vibrations. Another approach uses catalytic processes which synthesize ammonia from nitrogen and water without the use of elemental hydrogen by using various wavelengths of photoenergy. Consistent with these developments is the use of solar energy in various forms as the sole energy source and the use of water almost exclusively as the reducing agent. Exemplifying this latter approach are the following: "Photolysis of Water and Photoreduction of Nitrogen on Titanium Dioxide," Journal of the American Chemical Society 99:7189-7193 (1977) which describes the photoreduction of nitrogen to ammonia using titanium dioxide alone or when doped with iron, cobalt, molybdenum or nickel, or iron oxide alone; U.S. Pat. No. 4,113,590 which discloses the synthesis of ammonia and hydrazine by reduction of gaseous nitrogen with water using metal oxide catalysts under the influence of ultraviolet light; U.S. Pat. No. 4,427,510 which recites the synthesis of nitrogen-containing compounds by combining metal oxide compounds with gaseous nitrogen, a reducing agent such as water, and a source of light whose wavelengths are in the visible ranges provided by sunlight or artificial light; U.S. Pat. No. 4,612,096 which demonstrates the synthesis of ammonia from an aqueous medium using a solid metal oxide catalyst and an organic composition in the presence of photoenergy; and U.S. Pat. No. 4,762,600 which identifies a novel, activated catalyst for the synthesis of ammonia in a photoassisted reduction of molecular nitrogen by water.
A net result of the advances described within these publications (and the other references cited therein) has been the recognition and general acceptance of several premises as basic axioms in this art. These are: First, in any photopromoted catalytic process using a metal oxide catalyst for the reduction of molecular nitrogen, no measurable reduction of nitrogen will occur without some light energy being added to the reaction mixture. Second, water has been and presently remains the reducing agent of choice used almost always in the photopromoted catalytic synthesis; although other reducing agents such as aqueous organic suspensions have been recently developed, water remains the most favored reducing agent in such photopromoted catalytic syntheses. Third, the average yields of ammonia (and other nitrogen-containing compounds) using photopromoted catalytic processes now known in the art are notably small. Given these generally applicable axioms of this art, it will be apparent to one ordinarily skilled in this art that a photopromoted catalytic process for synthesizing ammonia using a solid metal oxide catalyst, molecular nitrogen, and molecular hydrogen as the reducing agent constitutes a major improvement and advancement in this field.